Two teams behind discovery of top quark

March 17, 1995

The March 2 announcement from Fermilab that the top quark has been discovered
was the culmination of a search that began 18 years ago and eventually involved
nearly a thousand scientists from more than a dozen nations.

Two experimental collaborations, of approximately equal size, working
independently and in fierce competition at Fermilab's Tevatron--the world's
most powerful proton-antiproton collider--produced consistent observations that
established the production and decay of top quarks. LBL scientists have been
prominently involved in both experiments from their inception and played key
roles in last week's landmark discovery.

The older of the two groups, which began its preparations in 1980, designed and
built the Collider Detector Facility (CDF) to conduct its experiments. The
second group, which began its work in 1984, designed and built the detector
array known as D-Zero (D0) to conduct experiments. Over the years, dozens of
LBL researchers have worked on each. Currently, the leader of LBL's CDF group
is Lina Galtieri and the leader of the D0 group is Ron Madaras. Both are senior
staff physicists in the Physics Division (PD). Another PD physicist, Bill
Carithers, is a visiting scientist at Fermilab, where he serves as
co-spokesperson for the CDF collaboration.

The CDF consists of three main components: a central detector system in the
middle of which collisions between protons and antiprotons take place, plus a
forward and backward detector to catch particles at small angles. The central
detector system weighs about 2,000 tons and features several types of
calorimeters for measuring energies, a Time Projection Chamber, a powerful
superconducting magnet, and large drift chambers. At the start of the project,
LBL was responsible for the hadronic component of the end-cap calorimeters--the
component that measured the energies of hadrons, subatomic particles that
interact through the strong force. The LBL end-cap hadron calorimeters were
completed and shipped to in 1985 Fermilab, where they were combined with the
rest of CDF's calorimeters to measure the total energy released when a proton
and antiproton collided. This information is necessary to select events with
the characteristic energy signature of top quark production.

More recently, physicists and engineers at LBL designed a sophisticated
microchip for the Silicon Vertex Detector (SVX), an extremely high resolution
instrument in the central CDF detector system that enables precise
identification and tracking of particle trajectories. There are 500,000
particle collisions per second occurring at the center of the CDF and the SVX
is able to analyze every one of them thanks to its innovative integrated
circuit design and readout electronics, which were developed at LBL.

"The microchip allows us to record the signals left by the particles that
traverse the four layers of silicon detectors in the SVX," says Galtieri. "This
allows us to reconstruct the point where the particles originated. For
particles which are decay products of b quarks, the origin is found to be
displaced from the point where the proton and antiproton have collided. The
presence of b quarks in conjunction with electrons from the W boson decay is a
powerful signature for the existence of top quark production."

Galtieri's group at LBL also developed a method to determine the mass of the
top quark, which they found to be approximately the same as an atom of gold.
This makes the top quark by far the heaviest elementary particle ever observed.
This analysis was based on a technique developed at LBL in the 1960s for bubble
chamber studies by the research group led by the late Nobel laureate Luis
Alvarez. Galtieri's group was able to extend the technique to handle much more
complex processes than it was originally designed for, including the production
of top quarks. Prior to this, many scientists did not believe it possible to
measure mass in such a complex process.

Last April, the CDF group announced the first evidence of the top quark. At
that time, CDF spokespersons said they had a strong case for the top quark but
lacked enough data to claim discovery. Since then, both the CDF and the D0
groups have tripled the amount of data collected and the threshold set by
particle physicists for discovery has been reached.

The D0 detector system consists of a compact tracking detector system, a
hermetic calorimeter, and a large solid angle muon detector system. The
innermost component of the central tracking detector system is the Vertex
Chamber, which was designed, built, tested, and commissioned at LBL. Tracking
detectors supplement calorimeters by measuring particle trajectories. Only when
trajectory and energy measurements are combined can scientists identify and
characterize particles. LBL's Vertex Chamber contains thousands of fine wires,
charged with different voltages, that can be used to locate the vertex of an
event--the precise spot within the beam pipe where a proton and an antiproton
collided. The chamber is divided into three independent layers, each of which
can measure the azimuth and longitudinal position of the tracks left by a
charged particle.

LBL was also responsible for designing, fabricating, and testing the D0 uranium
liquid-argon End Cap Electromagnetic (ECEM) calorimeters, which are used for
the precise energy and position measurement of forward electrons and photons.

"These are two of the most demanding pieces of apparatus in the D0 Detector,"
says Madaras. "Each ECEM is built as a single unit of monolithic multilayer
printed circuit signal board disks and uranium absorber disks with special
printed circuit boards that are used to read out 7,500 signals."

Like their CDF counterparts, the D0 collaboration also measured the mass of the
top quark and found it to be about 200 times the mass of a proton, slightly
larger than the CDF's measurement but consistent when the margins of error are
taken into consideration. For their mass measurements, the D0 group developed
its own multidimensional analysis of the patterns of top quark and W boson
decay.

The D0 group at LBL had a significant role in the development of the mass
analysis and, in addition, developed its own analysis of the shape of the
energy distributions in top quark and W boson decay," says physicist Tom
Trippe, deputy leader of the LBL D0 group. "This analysis augments the strength
of the conclusion that the events observed are due to top quark production and
decay."

LBL physicists say that both experimental groups intend to continue searching
for more examples of top quark production and decay until around the end of the
year. At that time, the Tevatron will be shut down for system upgrades that
will allow more rapid accumulation of data and a more complete understanding of
top production and decay. LBL's CDF group has also been involved in measuring
the mass of the W boson and has submitted a paper in which it reports the most
precise measurements ever made of the mass of both the top quark and the W
boson.

The W boson is one of two particles that carry the weak nuclear force.
Physicists believe that precise measurements of the top quark mass, along with
precise measurements of the W boson mass can provide information on the mass of
the Higgs boson, a hypothetical, extremely massive boson arising in the theory
of the electroweak force.

"The Higgs boson is now the only particle missing in the Standard Model," says
Galtieri. "It is necessary to explain why the mediator of the electromagnetic
force (the photon) has mass zero while the mediators of the weak force (the W
and Z bosons) have masses in the 80 to 90 GeV range."

If the Higgs boson is not within reach of the Tevatron's energies (as is most
likely the case), scientists will likely have to wait for the construction of
the European Large Hadron Collider at CERN. For this reason, Madaras says of
the top quark discovery, "I think that in particle physics, there will not be
anything like this for another decade."